Coated columbium alloy articles



United States Patent 3,206,289 CGATED COLUMBIUM ALDDY ARTICLES Elihu F. Bradley, West Hartford, Conn, and John I.

Rausch, Evanston, Ill., assignors, by direct and mesne assignments, to United Aircraft Corporation, East Hartford, Conn, a corporation of Delaware No Drawing. Filed Nov. 7, 1961, Ser. No. 150,628

2 Claims. (Cl. 29-497) This invention relates to a novel coating of columbium, cobalt, and nickel base alloys that will protect the base metal or alloy from oxidation in very high temperature environments. More particularly, this invention relates to electrodeposited coatings of aluminum on columbium, cobalt, or nickel base alloys, and a method for electrodepositing the aluminum on these alloys to produce compounds of the aluminum with the base metal of the alloys that provide an oxidation resistant coating for the alloys at very high temperatures.

The principal limitation in gas turbine technology today is the maximum turbine inlet temperature. The turbine inlet temperature is in turn limited by the temperature that the turbine vanes and blades are able to Withstand without danger of failure. The best available high temperature alloys in the prior art are nickel and cobalt base superalloys, but critical structural components, such as turbine vanes and blades constructed from such alloys are limited to maximum operating temperatures of between 1600 and 1900" F.

Among the technically most important physical qualities of columbium .as an alloy base are its high melting temperature (44-74 F.) and its low neutron-capture crosssection. Columbium is, therefore, potentially useful for fast aircraft and space flight vehicles and in nuclear reactors.

For many years it has been generally known that the high temperature strength properties of metals are closely related to their melting points. Thus, metals having a high melting point also tend to have high temperature strength potentials.

The need for structural materials for service at temperatures in excess of those obtainable with present materials of construction has stimulated interest in the refrac-. tory metals, chromium, columbium, molybdenum, and tungsten. Until about 1957, molybdenum was considered the chief prospect for such usage. However, at the high temperature service conditions needed, molybdenum oxidizes at a catastrophic rate, principally because molybdenum oxide is volatile at elevated temperatures. Be cause of the very great problems with coating HlOPYbdB'. num, interest has recently shifted to columbium as an alloy base for high temperature service.

' Columbium is inherently a soft, ductile, readily fabricable material. Although its melting temperature is about 4474 F., pure columbium becomes too weak for structural use at temperatures above 1200 F. Columbium is also a very reactive metal in that it dissolves large quantities of oxygen, and probably nitrogen, on exposure; .to atmospheres containing even small amounts of these elements at modest temperatures.

Although columbium suffersvfrom oxidation, its oxide does not volatilize, and it is thus potentially possible to localize oxygen attack on columbium by coating the metal.

Further advantages offered by. columbium base alloys as compared with molybdenum base alloys are that colum-. bium alloys are relatively more ductile and workable at low temperatures and columbium has a lower density than molybdenum. v 0 Until recent years, estimated ore reserves of colum bium were so small that there was only a mild interest in columbium base alloys. However, with the discovery I 3206,29 Patented Sept-14, 1965 ice of substantial ore bodies the potential availability of columbium has become so great that scarcity is no longer a restriction on its use.

The strength and oxidation resistance of pure columbium can be vastly improved by the addition of alloying elements. Copending application Serial No. 65,962, filed October 31, 1960, discloses and claims a number of different columbium base alloys that are suitable for use under high temperature service conditions about 2000 F. The coatings of the present invention may be used with such columbium base alloys. The coatings of the present invention are also useful with columbium base alloys earlier developed by others, such as Du Pont 31 (columbium-l0%molybdenuml0% titanium, by Weight).

The coatings and methods of coating of the present invention are not restricted to use with columbium base alloys, but may also be used with nickel and cobalt base alloys. The latter alloys are generally described as superalloys, and a number of them are well-known in the art of high-temperature technology. Indeed, these so-called superalloys are thought to have stretched the iron, nickel, cobalt group of alloys to the upper limits of possible highternperature applications. Both cobalt and nickel base alloys of the type discussed above can be successfully plated with the coatings and by the method of the present invention. The coatings and the method of this invention may thus be successfully used to deposit aluminum on a base metal in the form of aluminum compounds with a base metal that is a member of the group consisting of columbium base alloys, nickel base alloys and cobalt base alloys. In view of the foregoing, it is a primary'object of this invention to provide a coating on columbium, nickel, and cobalt base alloys that will protect these alloys from oxidation at the high temperatures at which they are designed to be used.

Additional objects of this invention are to provide a coating on columbium, nickel, and cobalt base alloys that, in addition to protecting the alloys from oxidation at high temperatures, is also capable of withstanding severe mechanical and thermal stresses, such as are encountered in jet engine service. The coating must also be compatible with the base metal of the alloy, so that the coating will not form low-melting phases or phase mixtures, volatile compounds, or a thick brittle layer. Also, the thermal expansion and mechanical properties of the coating must be reasonably matched to the base metal.

Further objects of this invention are to provide a coating for columbium, nickel, and cobalt base alloys that will provide good oxidation resistance at high temperatures through the formation of an adherent, continuous coating. The coating method provides some initial diffusion between the coating and the base metal but the resultant coating exhibits stability at operating temperatures to both oxidation and further diffusion. Also, the coating of-this invention is relatively ductile at all temperatures of use. Y

Other objects of this invention are to provide a coating on columbium, nickel and cobalt base alloys that will provide control of the uniformity of the coating and yield an essentially uniform coating on even intricately shaped parts and at the edges and corners of parts. The electrodeposition method of applying the'coatings of the present invention to parts formed from columbium, nickel, or cobalt base alloys provides a practical method for achieving uniformity in the coating.

A still further object of the present invention is to provide a method for electrodepositing aluminum on columbium, nickel, or cobalt base alloys to form a chemical compound of aluminum with a base metal of the 4 alloy that will provide an oxidation resistant coating for the columbium, nickel, and cobalt base alloys. The aluminum is electrodeposited on the base alloy from a molten salt bath.

The foregoing and other objects of this invention are realized by the coating of a columbium, nickel, or cobalt base alloy with an aluminum compound of the base metal that is resistant to oxidation. The coatings of this invention are useful for coatings on a base metal that is a member of the group consisting of columbium base alloys, nickel base alloys and cobalt base alloys that contain at least 40% by weight of columbium, nickel and cobalt, respectively. With columbium base alloys, the compounds formed are columbium aluminides, e.g., CbAl with cobalt base alloys, the aluminum compounds formed are cobalt aluminides, e.g., Co Al and CoAl, and with nickel base alloys, the aluminum compounds formed are the nickel aluminides, e.g., NiAl and Ni Al.

The coatings of the present invention protect columbium base alloys from oxidation at high temperatures by a diffusion coating consisting of columbium aluminides, such as CbAl at the outer surface that are bonded to a Cb-Al solid solution. The excellent oxidation re sistance of CbAl for example, is apparent from its oxidation rate. At 2000 F. this compound increases in weight, as the result of reaction with oxygen, at a rate of only 0.001 mg./mm. /hr. Specimens of columbium base alloys coated with aluminum by the method of this invention demonstrate a similar low oxidation rate at temperatures up to 2550 F.

Results of static oxidation tests on aluminum plated columbium at high temperatures indicate that a long life can be expected for columbium base alloys coated with the coating of this invention. The following results have been obtained.

Effective life of Temperature in F.: coating in hours 2550 25. 2375 More than 50. 2190 More than 170.

During high temperature exposure of coating specimens of columbium base alloys, the oxidation rate curve rises steeply during the first few hours but subsequently undergoes a sharp transition to a low rate of oxidation; the oxidation curve thus appears to be parabolic. Elim ination of, or a sharp decrease in, the rapid initial oxidation rate may be accomplished by treating coated columbium base alloy specimens in an inert atmosphere to diffuse the coating prior to exposure in air.

With the coatings of this invention, the last high temperature oxidation resistance is obtained within the range 2150 through 2550 F.

In the method of applying the coatings of this invention, the surface of the base metal may be prepared for coating in a conventional manner, but preferably it is prepared by degreasing with a conventional organic solvent then etched by dipping in a combined solution of nitric and hydrofluoric acid, and finally it is immersed in the molten salt bath for from two to five minutes to flux it before electrodeposition is commenced.

To carry out the plating operation, the specimen to be plated is wired as a cathode, inserted into a molten salt bath containing the electrolyte and a composition containing the plating metal. Current is then applied to a suitable anode and to the specimen to be plated as the cathode.

Aluminum may be deposited from a great variety of molten salt baths operated over the temperature range from 300 F. to 1900 F. For this invention, however, a bath operating Well above the melting point of alumimum is desired, and the baths used in the coating process are preferably operated within a temperature range of from 1350 to 1900 F. The molten salt baths are also preferably based on the system NaF-AlF with A1 0 used as the electrolyte. In this NaF-A11 system, two

l eutectics occur: one at 1625 F. and 23% AlF and the other at 1265" F. and 63% by weight AlF The higher melting mixture may be made most conveniently by blending NaF and cryolite (Na AlF and the lower melting mixture by combining AIR and cryolite (Na AlF Bath compositions having greater than 63% by weight A1F have been found impractical to prepare, because the volatility of the mixture of cryolite with aluminum fluoride (A11 increases too rapidly with increase of the aluminum fluoride content above 63%.

The bath compositions of this invention used to coat columbium, nickel, or cobalt alloys thus comprise cryolite (Na AlF and A1 0 and may optionally include one or more compounds selected from the group consisting of NaF and AlF When the bath comprises cryolite and A1 0 the range of cryolite may be from to 95% by weight and the range of A1 0 may be from 5 to 25 o by weight. Preferably, the bath comprises to by weight cryolite and 10 to 20% by weight A1 0 The preferred operating temperature for these baths is from 1600 to 1900 F.

When a higher melting mixture is desired, the bath may comprise from 55 to 65% by Weight of cryolite, from 15 to 35% by weight of NaF and from 5 to 25% by weight of A1 0 Preferably, the bath comprises about 60% by Weight of cryolite, 20 to 30% by weight NaF, and 10 to 20% by Weight of A1 0 The preferred operating temperature for these baths is from l600 to 1800 F.

When a lower melting mixture is desired, the bath may comprise from 50 to 75% by weight of cryolite, and from 15 to 40% by weight of MR, and from 5 to 15% by Weight of A1 0 Preferably, the bath comprises 55 to 70% by Weight of cryolite, 20 to 35% by Weight of MB, and about 10% by weight of Al O The preferred operating temperature for these baths is from 1350 to 1700 F.

The bath may also comprise all four compounds in the ranges of from 65 to 75% by Weight of cryolite, 5 to 20% by weight of NaF, and 5 to 20% by weight of All-"' and 5 to 15 by weight of A1 0 A preferred proportion of compounds for such a bath is 70% by Weight of cryolite, 12% by weight of NaF, 8% by weight of AlF and 10% by weight of A1 0 To supply current for operation of the plating process, any conventional DC. power source may be used, such as a 0 to 20 volt rectifier with a plating circuit consisting of copper lead wires connected directly from the rectifier to the anode and cathode.

Anodes suitable for use with molten salt baths may be used, preferably, however, the anode comprises a graphite crucible that also serves as a container for the bath.

Current densities may vary from 0.15 to 3.0 amps/cm and preferably are Within the range 0.15 to 1.5 amps./ cm. Plating time may vary from 2 to 90 minutes or more depending upon the thickness of coating desired. Preferably, a plating time of from 5 to 40 minutes is used.

Upon completion of electrodeposition, the specimens being plated will be found to be coated with a layer of salt comprising mainly cryolite and NaF. These salts should be removed, and this can be accomplished in a variety of ways. A preferred technique is to place the specimen in a molten bath comprising a mixture of sodium chloride and lithium chloride and operated at a temperature of approximately 1200 F. This results in substitution of the fluoride salts with chloride salts, and the chloride salts can be easily removed by ultra-sonic cleanby Weight ing under water or by light brushing. Another preferred technique is to immerse the specimens in a solution of chromium oxide in dilute phosphoric acid and subject them to ultrasonic vibration.

Care should be taken in removing the salt to ensure that the coating itself is not harmed.

In a preferred form of the invention, electrodeposition of aluminum onto columbium base alloys is conducted at 1800 F. and a current density of 1 amp/cm. for 10 minutes. Also, it is desirable to subject the columbium base alloys plated with aluminum using the process of this invention to diffusion in air at 1600 to 1300. F. after plating for a period of from one to eight hours. Preferably, the specimens are diffused at 1700 F. for four hours.

For a clearer understanding of the invention, specific examples of the invention are given below. These examples are merely illustrative and are not to be understood as limiting the scope and underlying principles of the invention.

Example I A protective oxidation resistant coating of columbium aluminide (CbAl on a columbium base alloy, as an example of this invention, was prepared by electrodeposition of aluminum on columbium from a molten salt bath having the following composition:

Percent by weight N21 AlF (cryolite) 62.5 NaF 21.0 A1 0 (alumina) 16.5

The bath was operated at a temperature of 1800 F. and electrodeposition occurred at current densities varying between 0.1 and 0.3 amp./cm. A graphite crucible was used both as a container for the bath and as the anode. A specimen of a columbium base alloy to be coated was made the cathode of the electroplating apparatus. The aluminum was then deposited directly on the columbium base alloy cathode.

A plating time of thirty (30) minutes at an average current density of 0.15 amp/cm. caused an increase in thickness on the columbium base alloy of approximately 0.0003 in. or 0.3 mil on each surface of the specimen that was plated. This increase in thickness of the plated specimen was accompanied by an increase in weight of about 3 mg./cm. The aluminum was found to have penetrated into the specimen to a maximum depth of 2 to 4 mils.

Surface preparation of the columbium base alloy that was plated comprised degreasing in a conventional manner and etching in a solution of 2I-INO :1I-IF:1H O. Further removal of oxides or nitrides from the surface of the alloy was accomplished by immersing the specimen in the molten salt bath for two (2) to five (5) minutes prior to plating. At a temperature of 1800 F., the molten salt bath effectively removed films from the surface of the columbium'base alloy by solution of oxygen and nitrogen in the bath itself.

When the plated specimens of the columbium base alloy were removed from the molten salt bath, they were found to be coated with a layer of salt comprising mainly Na AlF and NaF. These salts had to be removed before the sample could be exposed to an oxidizing environment at temperatures above 1800 F., or the salt would dissolve the coating at these high temperatures.

Two different methods, both effective, were used to remove the salt. In one method, the specimens were placed in a mixture of NaCl and LiCl operated at 1200 F.; this dissolved the fluoride salts. The substituted chloride salts were then easily removed either by ultrasonic cleaning under water, or by light brushing.

In the other method, the samples were immersed in a solution of 20 g. of CrO +27 ml. H PO +973 ml. H 0 and subjected to ultrasonic vibration.

Static oxidation tests on the samples thus produced resulted in an oxidation resistant life varying from more than 170 hours at 2190 F. to 25 hours at 2550" F.

Example 11 Specimens of columbium base alloys coated with aluminum to provide a columbium aluminide (CbAl coating on the specimens were prepared in the same manner as described in Example I, except that the molten salt bath comprised the following composition:

Percent by weight 0 Na AlF (cryolite) 7 NaF, 12 AlF 8 A1 0 (alumina) 10 All other conditions were the same as those described under Example 1. Test results on the coated samples of the columbium base alloy thus produced were also the same as those given for Example I.

' Example III Percent by weight Na AlF (cryolite) 55 AlF 35 A1 0 (alumina) 10 In this instance the bath was operated at a temperature of 1600" F., but otherwise all conditions remained the same as for Example I. The samples of coated columbium base alloys produced according to Example III also yielded the same test results as those obtained for samples prepared as described in Example I.

Example IV A cobalt base alloy containing by weight 20% chromium, 20% nickel, 4% molybdenum, 4% tungsten, 4% columbium, and the balance cobalt in the form of a wrought ingot divided into approximately one (1) inch square specimens was plated with aluminum in a molten salt bath of the following composition:

Percent by weight Na A1F (cryolite) 59 A1123 A1 0 (alumina) 2 The bath was operated at 1500 F. and a current Example V An as cast ingot of a cobalt base alloy containing by weight 21% chromium, 11% tungsten, 1.75% columbium, 2% iron, and the balance cobalt, was divided into approximately 100 one (1) inch square specimens. These specimens were plated as described in Example IV. The test results obtained on these specimens were the same as those for Example IV.

The present invention in its broader aspects is not limited to the specific compositions and examples described, but also includes within the scope of the accompanying claims any departures made from such compositions and examples which do not sacrifice its chief advantages.

What is claimed is:

1. An article of manufacture having good stress-rupture strength at high temperatures and high-temperature oxidation resistance, which comprises a core of metal selected from the group consisting of columbium and columbium-base alloys, the articles having a ditfusionally stable, mechanically stress resistant and oxidation resistant surface zone consisting essentially of CbAlg.

2. An article of manufacture having good stress-rupture strength at high temperatures and high-temperature oxidation resistance which comprises a core of metal se- 'lected from the group consisting of columbiurn and colurnbium-base alloys, the article having a surface layer consisting essentially of CbA1 and a sublayer consisting essentially of a Cb-Al solid solution, the sublayer being between the surface layer and the core.

References Cited by the Examiner UNITED STATES PATENTS 2,682,101 6/54 Whitfield et a1 29198 2,752,667- 7/56 Schaefer et a1 29-198 8 Charlton et a1 204-39 Sibert 20439 Russell 29-1962 Boggs 29194 Hanink et a1. 11751 Russell 29-196.2 Moller 20439 Cox 29197 X Hirakis 29198 Carlson 29-497 X DAVID L. RECK, Primary Examiner. MURRAY TILLMAN, Examiner. 

1. AN ARTICLE OF MANUFACTURE HAVING GOOD STRESS-RUPTURE STRENGTH AT HIGH TEMPERATURES AND HIGH-TEMPERATURE OXIDATION RESISTANCE, WHICH COMPRISES A CORE OF METAL SELECTED FROM THE GROUP CONSISTING OF COLUMBIUM AND COLUMBIUM-BASE ALLOYS, THE ARTICLES HAVING A DIFFUSIONALLY STABLE, MECHANICALLY STRESS RESISTANT AND OXIDATION RESISTANT SURFACE ZONE CONSISING ESSENTIALLY OF CBAL3. 